Multiphoton photochemical processes generally involve the simultaneous absorption of two or more photons by an absorbing chromophore. In such processes, the chromophore typically does not absorb at the wavelength of the individual photons, but at sufficiently high intensity a simultaneous absorption of multiple photons by the chromophore occurs. For example, simultaneous absorption of two photons having a wavelength λ has the effect of absorption of a single photon of wavelength λ/2. While single-photon absorption generally scales linearly with the intensity of the incident radiation two-photon absorption scales quadratically, and higher-order absorptions scale with a corresponding higher-order power of the intensity of the incident radiation. As a result, it is typically possible to perform multiphoton curing processes with three-dimensional spatial resolution. Furthermore, since the incident radiation is not attenuated by single-photon absorption within a photoreactive (e.g., polymerizable) material, it is generally possible to selectively excite molecules at a greater depth within the material than would be possible via single-photon excitation. In multiphoton imaging processes, a layer of unexposed photoreactive material disposed on a substrate is commonly referred to as a “photoresist”.
Multiphoton-induced photopolymerization, typically using a femtosecond pulsed laser (e.g., an infrared laser), has been used to fabricate three-dimensional devices with sub-micron resolution. Multiphoton fabrication has been used to manufacture mechanical and optical devices, such as cantilevers, gears, shafts, and microlenses.
In typical multiphoton imaging processes, exposed regions of a photoresist crosslink and harden. Solvent development removes unexposed (non-polymerized) regions of the photoresist, leaving behind an imaged three-dimensional structure. Depending on the particular photoresist, this development step may lead to distortion of the three-dimensional structure.
A common method used in the art of multiphoton microfabrication utilizes a solvent cast solid polymerizable composition (sometimes referred to as a “photoresist”). Imaging such solid polymerizable compositions is complicated by either: 1) the existence of an air interface between the photoresist and the optics used to focus the laser beam; or the presence of an index matching fluid sandwiched between the optics and the surface of the photoresist.
In practice, the resolution achieved in multiphoton fabrication processes is typically dependent on the size and shape of voxels (i.e., volumetric pixels). In the case of multiphoton-induced reaction of a photoreactive material, the term “voxel” refers to the smallest volume element of reacted photoreactive material that is generated by the multiphoton induced reaction.
By repetition of the voxel, three-dimensional objects may be constructed in analogous manner to the way a two-dimensional object is constructed from pixels. The maximum resolution of a three-dimensional object manufactured in this way is generally limited by the size and shape of the voxel. In addition to voxel shape, the overall volumetric polymerization rate of multiphoton fabrication processes are important for them to be useful in a practical (e.g., commercial) setting where rapid throughput is desirable.
There is a continuing need for materials and methods suitable for use in multiphoton imaging processes that can be used to fabricate sub-micron three-dimensional features.